METAMORPHISM 381 
Let us suppose that the cavity which is to be the 
seat of deposit, is a fissure or fault plane, leading deep 
down into the earth; for it is in such cavities that we 
discover the most valuable ore beds. Water is enter- 
ing this fissure from some region 
where it has obtained a high tem- 
perature. Passing upward through 
the cavity, the water becomes less 
highly heated, and finally, reaching 
the surface, flows out as a hot spring 
(Fig. 229 and p. 362). 
As it goes on its journey, the 
water is able to dissolve mineral 
substances because of its high tem- 
perature. By this means it takes 
from the rocks some elements which 
transform it to the condition of a 
powerful acid, or of an alkali; and 
z J Diagram to _ illustrate 
thus strengthened, acting upon min- wold: Listenin ok 
fault plane filled with 
water. Intensity of 
the water is able to take these in solu- shading indicates high 
temperatures. 
erals which contain valuable metals, 
tion in some chemical combination. 
Passing upward, the water cools; as it loses heat, it 
has less power of solution, and begins to deposit. It 
may hold several substances in solution,— such, for 
instance, as quartz, calcite, and an ore of iron. At 
one time or place it deposits quartz on both walls of 
